The objective of an antilock brake system (ABS) in a vehicle is to reduce the brake pressure in the different wheel brakes in case of excessive braking so that the wheels generate maximum brake force without locking. This aids in retaining vehicle stability and steerability while providing shorter stopping distances.
In vehicles capable of both hard-coupled four-wheel drive or uncoupled two-wheel drive operation, and having antilock brake systems, the braking control characteristic of the ABS procedure is highly dependent on the mode of operation utilized in the drivetrain. This dependence results, in part, from the inherent differences in stability, steerability, and stopping distance between two-wheel and four-wheel drive operation. For example, the four-wheel drive mode inherently provides improved braking ability (i.e. shorter stopping distances). However, the four-wheel drive mode produces degraded cornering ability under lock-up conditions compared to the two-wheel drive mode.
In some prior art four-wheel drive vehicle equipped with a standard ABS which controls the front and rear wheels independently, the torque inputs at the front and rear drive shafts may be unequal during braking due to the ABS independent modulation of the front and rear wheels. This causes torque interference and torsional vibration of the drive shafts and unanticipated torque transfer over the brake torque. Consequently, the following problems can occur, especially on slippery (or low-mu) driving surfaces:
(i) the reference wheel speed, which estimates the actual vehicle speed based on the four measured wheel speeds, can significantly underestimate the actual vehicle speed since all four wheels may lock together.
(ii) once the wheels experience high slippage, recovery of the wheels' speed to the vehicle speed becomes slow because of the greater moment of drivetrain inertia; and
(iii) torsional vibration of the drive shafts occurs with independent modulation of the front and rear wheels and unanticipated torque transfer over the hydraulic brake torque.
Various solutions for modifying the ABS control algorithm due to the specific requirements of four-wheel drive operation have been proposed and implemented. However, the benefits gained by these modifications cannot be realized without an accurate indication of the mode of operation of the drivetrain.
Vehicles whose differentials can be either locked or free (thus providing either four-wheel or two-wheel drive) include means of indicating the commanded or intended mode of operation of the drivetrain. Such instrumentation is subsequently referred to as a drivetrain indicator. One example of a drivetrain indicator is a dashboard light which is "on" when the vehicle is commanded or intended to be in a four-wheel drive mode of operation and "off" for an intended two-wheel drive mode of operation. In such vehicles, an electrical or electro-optic sensor is used to determine the status of the dashboard-light drivetrain indicator, and this status is relayed to the antilock brake system. Another example of a drivetrain indicator is a flag variable within a software routine. Regardless of the form of the drivetrain indicator, the antilock brake system monitors this indicator and modifies its braking control algorithm in response to the intended mode of operation.
The major shortcoming of modifying the antilock brake system control algorithm based simply on monitoring the drivetrain indicator is that the actual mode of operation is not necessarily the same as the intended or commanded mode of operation. For example, a vehicle may be operating in an uncoupled two-wheel drive mode, and then commanded to operate in a hard-coupled four-wheel drive mode. In this example, the dashboard light could indicate the intent of hard-coupled four-wheel drive mode operation for a period of time before the drivetrain has indeed entered the hard-coupled state. Further, this indicator can yield false indications in vehicles with manually locking wheel hubs when the wheel hubs are unlocked.
The need exists for accurately determining whether a four-wheel drive mode of operation indicated by the drivetrain indicator is actually a false indication, so that characteristics of the ABS procedure can be modified for the actual two-wheel drive operation of the vehicle.
A crude method for detecting false four-wheel drive operation exists in prior art systems. The method requires both front wheels to be in slip with the rear not in slip for an extended period of time. Also, if the front departures are out of phase, this method will not detect false four-wheel drive operation.